Heretofore, naphtha, so-called fossil material has been used as a material of a polyamide resin. However, to prevent global warming by controlling emission of carbon dioxide and to promote a recycling society, it has been expected to replace a material for polyamide resin production with a biomass-derived material.
As a polyamide resin produced by using a biomass-derived material, 56 nylon has been known. As a method for producing 56 nylon, a method of heat polycondensing diaminopentane and adipic acid (e.g. Patent Document 1), a method of preparing a salt of diaminopentane and adipic acid and heat polycondensing it (e.g. Patent Document 2), etc. have been proposed.
56 nylon thus produced has heat resistance and mechanical properties substantially equal to those of 6 nylon and 66 nylon but is inferior in lasting thermal stability. A resin which is poor in lasting thermal stability is unstable for a large molded product which requires a long forming cycle, such as a resin intake manifold which is an automobile component. Further, it is also unstable for an extruded product such as a film or a filament, since granular defects called fisheyes are likely to generate.
Patent Document 3 proposes to improve lasting thermal stability by blending 56 nylon and 66 nylon. However, the biomass ratio of the material in the polyamide resin (the proportion of the biomass-derived material in materials used for the polyamide resin) decreases in an amount corresponding to the amount of 66 nylon.
Under these circumstances, a polyamide resin having high lasting thermal stability and a high biomass ratio has been desired.    Patent Document 1: JP-A-2003-292612    Patent Document 2: U.S. Pat. No. 2,130,948    Patent Document 3: JP-A-2006-348057